1. Field of the Invention
This invention relates to an automatic diaphragm device in an interchangeable lens for a single lens reflex-type camera which has the diaphragm fixedly positioned with respect to the camera mount portion.
2. Description of the Prior Art
Conventional interchangeable lenses for cameras have adopted diaphragm blades of the type which are movable with the picture-taking lens group. However, in lenses of long focal length, the distance between the diaphragm position and the camera mount portion is long so that the amount of axial movement required of the lens during distance adjustment is great, and in brighter lenses, the aperture diameter is greater and the number of parts associated with the diaphragm is larger. For these reasons, in lenses of this kind automatization of the diaphragm also presents difficulties in terms of weight if made in accordance with the teachings of the prior art.
Even in such lenses, if the diaphragm position is fixed with respect to the camera mount portion, the number of the parts associated with the diaphragm may be reduced to permit lighter weight of the diaphragm device and this will be a great advantage in automatization of the diaphragm. However, when the diaphragm position is so fixed, the quantity of light reaching a film surface, which is determined by the diaphragm position, will vary with the axial movement of the lenses because of the different effective light beam diameters of the lenses disposed forwardly and rearwardly of the diaphragm. It then becomes necessary to effect aperture correction and obtain a predetermined F-value in order to provide an aperture diameter corresponding to the variation in the amount of light which would reach the film surface.